780 BECORD OF CURRENT RESEARCHES RELATING TO 



original article that " witli high powers and wide angles, the agree- 

 ment between his oion instrument and that of Professor'Ahhe is as close 

 OS could he desired," while Colonel Woodward shows * the source 

 of the erroneous readings which Professor Smith obtained when he 

 attempted to use his apparatus after the Abbe method with low-power 

 objectives. 



Mr. Wenham, writing on this paper,! calls attention to the follow- 

 ing experiments, not as indicating any defect in Professor Smith's 

 principle, but in the mode of use which he considers may be the cause 

 of error in the measurement : — 



In order to well define the transmitting diameter or boundary of 

 the light spot on the front of the glass. Professor Smith applies a piece 

 of tissue paper or ground glass to the open end of the object-glass tube. 

 This gives results in excess of truth, which may be attributed to the 

 diameter of the screen. Instead of following this course, set a lamp a 

 few feet away exactly in the axis of the object-glass, and focus the 

 Microscope on the index line ; the flame of the lamp will be beauti- 

 fully in focus at the same time, showing that the examining Microscope 

 does not in the least alter the focal distance of the object-glass under 

 test. On traversing the lower Microscope sideways, the flame appears 

 curiously projected, as if it were actually strung on to the lines ruled 

 on the glass. At an obliquity greater or less according to the aperture 

 of the objective, the flame shows a tendency to leave the line before it 

 vanishes. Now, with the flame, this limit gives a result less than 

 when the ground glass is used. The former is, therefore, the limit 

 of aperture. The ground glass does not increase the transmitting 

 diameter of the front, which remains the same under all circum- 

 stances ; but it allows oblique rays from a screen or field of view to 

 pass through. The lamp flame is distant, and in one fixed, axial, 

 focal point. As the image of the flame and the ruled line are kept in 

 the direction of the axis of the examining Microscope, the angular 

 traverse of the line of this axis must indicate the true aperture. 



Mr. Wenham also thus summarizes the facts upon which Pro- 

 fessor Smith's conclusions are based : — 



" 1. That the front lenses of Microscope object-glasses only admit 

 incident rays through a central area, far within their actual diameter. 



" 2. That angle of aperture strictly means the angle measured by a 

 triangle taken from the extremities of the diameter of this light sjiot 

 as a base line up to the focal distance in the axis, whether that 

 distance is in air, water, or glass, with the difference of angle due to 

 the refraction of each. 



" 3. That rays extending laterally from places without the central 

 focal point do not constitute a proper angle of aperture, but cover an 

 area known as field of view. 



" 4. That rays from every part of the field of view pass through 

 every portion of the transmitting diameter of the front lens, and, 

 together, enter the pujiil of the eye from the eye-piece. 



" 5. That in the optical methods of measuring angles of aperture 



* ' Ain. Quart. Micr. Journ.,' i. (1879) p. 27S. See infra, p. 734. 

 t Ibid., p. 280. 



